U.S. patent number 5,481,105 [Application Number 08/071,170] was granted by the patent office on 1996-01-02 for neutron backscatter gravel pack logging sonde with azimuthal scan capability.
This patent grant is currently assigned to Halliburton Company. Invention is credited to Randy Gold.
United States Patent |
5,481,105 |
Gold |
January 2, 1996 |
Neutron backscatter gravel pack logging sonde with azimuthal scan
capability
Abstract
An apparatus (sonde) and method of measuring density, or gravel
pack quality, in a cased well borehole using a fast neutron source
and one or more thermal neutron detectors is described. In one
embodiment, a neutron source creates a fast neutron flux which
reacts primarily with the material within the borehole casing while
a collocated neutron detector counts the number of backscattered
thermal neutrons. A novel means of obtaining azimuthal measurement
discrimination is provided by a rotating neutron shield. In one
instance the shield is quite substantial, creating a narrow
measurement window. In another instance, the shield only marginally
screens the detector, creating a large measurement window. In an
alternative embodiment, a second thermal neutron detector is spaced
distally from the neutron source and first detector. This second
detector is used to provide a measurement of the borehole's
background, or environmental neutron activity, and can be used to
improve the quality of the sonde's gravel pack density
measurement.
Inventors: |
Gold; Randy (Houston, TX) |
Assignee: |
Halliburton Company (Dallas,
TX)
|
Family
ID: |
22099714 |
Appl.
No.: |
08/071,170 |
Filed: |
June 4, 1993 |
Current U.S.
Class: |
250/266; 250/265;
250/269.4 |
Current CPC
Class: |
E21B
43/04 (20130101); G01V 5/107 (20130101) |
Current International
Class: |
G01V
5/10 (20060101); G01V 5/00 (20060101); G01V
005/10 () |
Field of
Search: |
;250/269,370.1,270,264,265,266 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Fields; Carolyn E.
Attorney, Agent or Firm: Arnold White & Durkee
Claims
I claim:
1. A method for making azimuthally sensitive measurements of a
gravel pack along a cased well borehole comprising the steps
of:
in a cased well borehole having a gravel pack therein on the
exterior of a tubing string, positioning a sonde supporting a
source of neutron radiation and a first detector for detecting
neutron radiation from the gravel pack region around the tubing
string for producing an output signal indicative of the neutron
radiation detected by said first detector, wherein said first
detector is partially shielded in azimuth, and is rotated so that
detected radiation is azimuthally measured;
providing neutron irradiation from the neutron source to interact
with the gravel pack material around the tubing string to produce
neutron radiation to enable detection of the neutron radiation with
said first neutron radiation detector; and
altering the azimuthal direction of sensitivity of said first
neutron radiation detector to sweep a desired azimuthal area with
respect to said first detector.
2. The method of claim 1 wherein said sonde comprises a second
neutron radiation detector for detecting neutron radiation from
said gravel pack region around said tubing string, where said first
detector is shielded in azimuth for rotational measurement and said
second detector is azimuthally uniform in detection.
3. The method of claim 2 including the step of shielding the second
detector from thermal neutron flux.
4. The method of claim 3, wherein said first detector's shield has
an azimuthal window with an angle of up to about 45.degree..
5. The method of claim 2 including the step of shielding said first
detector fully therearound with an azimuthal window.
6. The method of claim 2 including the step of relatively rotating
the first detector's shield with respect to a tool axis extending
along the cased well borehole.
7. The method of claim 6 including the step of rotating the first
and second detectors.
8. The method of claim 1 including the step of forming radiation
from said source and measuring the first detector's response as a
function of well depth.
9. A method of making azimuthally sensitive measurements of a
gravel pack; along a cased well borehole, comprising the steps
of:
in a cased well borehole having a gravel pack therein on the
exterior of a tubing string, positioning a sonde supporting a
source of neutron radiation and first and second detectors for
detecting neutron radiation from the gravel pack region around the
tubing string for producing an output signal indicative of the
neutron radiation detected by said first and second detectors,
where said first detector is shielded in azimuth for rotational
measurement and said second detector is azimuthally uniform in
detection, with said first detector shielded with a shield of up to
about 75.degree. azimuthal angle;
providing neutron irradiation from the neutron source to interact
with the gravel pack material around the tubing string to produce
neutron radiation to enable detection of the neutron radiation with
said first neutron radiation detector; and
altering the azimuthal direction of sensitivity of said first
neutron radiation detector to sweep a desired azimuthal area with
respect to said first detector.
10. The method of claim 9 including the step of shielding the
second detector from thermal neutron flux with a uniform shield
therearound.
Description
BACKGROUND OF THE DISCLOSURE
The present invention is directed to an apparatus and related
method for obtaining an azimuthally directed measurement in a cased
well borehole and more particularly in one provided with a
production tubing surrounded by a gravel pack on the exterior of
the production tubing and on the interior of the casing. It is
particularly useful for wells into formations which are produced in
this fashion, namely, by positioning a casing in the well borehole,
cementing the casing in the well and subsequently forming
perforations through the casing into the formation so that
formation fluid production is obtained. In many wells, one problem
is that there will be excessive sand production from a producing
formation, and that is often countered by installing a gravel pack
in the cased well. A typical arrangement involves a production
tubing string centralized within a casing cemented in place with a
gravel pack and sand screen supporting the gravel pack on the
interior of the casing.
Gravel packing is performed to keep loosely compacted formations
from eroding during production. Formation on erosion generally
begins at or near the perforation tunnels where fluid flow
velocities are highest. When this type of erosion occurs, there are
several possible detrimental results such as formation fines which
plug the formation and reduce or stop production; they may fill the
casing stopping production, and they may be carried by the
production stream where they can cause a variety of equipment
damage.
The idea behind gravel packing is to fill the perforation tunnel
with a permeable material which reduces the flow velocity. It is
also desirable that this packing be of a similar pore size to the
formation in order to further reduce the movement of formation
fines. In the event that the perforation tunnel portion of the
gravel pack is not completely successful, the annular portion of
the pack inside the casing may act as a barrier to filter the
formation material from being carried downstream by the fluid
flow.
It is very difficult to measure how well the perforation tunnels
are packed. However, much can be learned about the quality of the
packing procedure by measurements which detect the uniformity of
the annular portion of the pack inside the casing. It is desirable
to detect both increases and decreases in gravel pack porosity
which may indicate voids and plugging respectively.
Immediately after performing a gravel pack procedure, before
flowing the well, voids in the annular area and inside the casing
may indicate that the perforation tunnels were not sufficiently
packed. It also, belies later problems in that even if the tunnels
are well packed, the annular void provides a location for the
flowing fluid to carry the pack material from the perforation
tunnel into the casing thereby unpacking the tunnel. After a well
is produced and there is a partial failure of the packing (some or
many tunnels are not packed), the annular portion of the pack acts
as a filter to prevent formation fines from moving downstream.
Voids detected at this time indicate the reduced capability of this
filtering material. This type of failure may also be indicated by
the reduced packing porosity, production fluids into the casing at
high flow velocities, they will actually erode the gravel pack
screen itself if it is not protected by the annular portion of the
pack. A work over is necessitated to correct the pack. Work overs
interrupt production and cost substantial sums of money to provide
service to a well. Even then, when the work over is complete, the
pack in the well may sand up again.
In conjunction with gravel pack, a screen typically will be
installed, namely a screen formed of screen wire or screen cloth
which is inserted in the well borehole to prop up the gravel pack.
This defines an annular support for the gravel. This is highly
desirable to extend the life of a well.
It is possible to locate a void in the gravel with a tool which is
responsive to density. Consider for instance a density measuring
device where there is a substantial contrast between the fluid in
the pores and the gravel. The fluid may have a density of about
1.0, but perhaps slightly more if it is salt water, and the gravel
pack material might have a density of about 2.65 gm/cc. A loss of
gravel pack material in a particular region will alter the
matrix/fluid ratio and thus reduce the measured bulk density.
Conversely, a pack plugging with formation fines will have an
increased density. The density is inversely proportional to the
detected count rate of a typical gamma ray fluid density tool used
in this circumstance and can be employed to indicate gravel pack
quality.
That type measurement is made all the more difficult as a result of
recent advances which have been introduced for gravel pack
materials. The contrast in the density of the matrix and fluid has
been reduced with the advent of new packing materials. Regrettably,
this makes measuring gravel pack quality more difficult. In other
words, as the specific density of the matrix material decreases
from a typical density of 2.65 down to 2.0 or perhaps even less,
the loss in contrast in the density measurement between the matrix
material and the pore fluid makes measurement the gamma density
approach difficult, perhaps almost impossible. The present
disclosure sets forth a method and apparatus which can be used to
measure gravel pack quality that does not depend on a contrast
between the fluid and matrix material densities.
BRIEF DESCRIPTION OF THE INVENTION
The present disclosure is directed to a sonde having an external
housing which is adapted to be lowered into a well borehole on a
logging cable. It is intended to be operated in a centralized
position. Moreover, it incorporates a neutron source which is
installed at a zero spaced detector to accomplish the measurement.
Additionally, a second detector which is located remotely from the
first detector can be used. The detectors cooperate with a neutron
source capable of forming a neutron flux directed into the
formation in the vicinity of the source where the neutrons react
with the respective materials and are back scattered toward the
detector. This relies on neutron back scatter as opposed to forward
scattering and absorption which is involved in porosity
measurements. With the source located at the center of the
detector, fast neutrons leaving the source are back scattered to
the detector only if they undergo large angle scattering to be
returned to the detector. At this point the neutrons are generally
at low energies, thermal/epithermal can be detected by an
appropriate detector. The interaction with the environmental
materials primarily involves the neutron back scatter and
absorption in contrast with forward scattering and absorption
involved in porosity measurements. The present system thus takes
advantage of a zero spaced neutron source located at the center of
the detector. A flux of fast neutrons from the source thus require
the large angle back scattering for return to the area of the tool
in the well borehole to interact with the detector. The detected
neutron flux is predominately effected by the gravel pack material
in the cased well. The constituent materials of the environment on
the exterior of the casing are substantially not involved in the
reaction yielding the detected back scatter neutrons. The primary
reaction of value is the elastic collision with hydrogen nuclei
which are found in the fluids in the spaces between the matrix
material of the gravel pack. The detected count rate is thus
proportional to the hydrogen content or porosity of the environment
surrounding the source, and most especially is at such a distance
or range from the source that other materials are not involved,
thereby excluding neutron interaction with the formations to the
exterior. This takes advantage of the fact that a typical thermal
neutron diffusion path or length is typically just a few inches,
not much more than about five inches, or even less. Thermal
neutrons existing at greater distances are ultimately absorbed and
not detected. This practically limits the range of investigation.
This enables a zero spaced detector to be appropriately sensitive
to the hydrogen nuclei in the immediate vicinity and therefore
sensitive to and responsive to the pore fluid in the gravel pack
matrix. The response is substantially insensitive to other
materials beyond that area.
The source to detector spacing controls the effective radial depth.
By use of a second detector which is axially aligned in the sonde
but at some distance from the zero spaced neutron source in the
first detector just mentioned, data can be obtained from the second
detector which enables compensation for environmental effects. It
provides a base line enabling a measurement which can then be
employed with the readings of the zero spaced detector to
especially eliminate such effects.
There is an additional factor involved in detection of thermal
neutrons. The count rate of the detector is in part determined on
the extent of materials which are thermal neutron absorbers. This
includes elements such as chlorine or boron. With an unintended
substantial increase in such absorbing elements, it is possible to
have a false reading indicating gas or a tighter packing of the
gravel pack than is actually resident in the area. It is helpful to
provide a correction to account for variations in the thermal
neutron absorbing elements. In other words, if there is a uniform
distribution of absorbing elements, the log can be calibrated to
take this fact into account. If however variations arise from
variations in absorbing elements, that fact needs to be recognized
and removed from the data before determination of the log quality.
To accomplish this it may be helpful to incorporate a neutron
detector which is sensitive to neutrons having energies above the
thermal level. This can be accomplished simply by wrapping the
detector element with a cadmium shield which will eliminate thermal
neutron flux, thereby providing a detector which is far less
sensitive to thermal neutrons and hence far less sensitive to the
presence of thermal neutron absorbing elements. That can be used to
provide a measurement where the count rate corrects for the
variations in absorbing materials. In addition to cadmium, other
suitable materials are samarium, carbon, gadolinium and boron. It
is one advantage of the present apparatus to incorporate a shield
with a detector subject to rotation about the detector to vary the
azimuthal neutron flux impingement on the detector. Consider as an
example a shield around the detector which encircles the entire
detector save and except a lengthwise window of specified width,
for example something between 10.degree. and 30.degree. . The
shield markedly cuts down the count rate. Assuming that the count
rate remains sufficiently high to have some degree of statistical
reliability, the shield when rotated provides an azimuthal response
indicative of the directional orientation. This enables
determination of voids in the gravel pack matrix as a function of
direction with respect to the axis of the logging tool. Moreover,
the window in the shield permits a response which includes thermal
neutrons as well as those of higher energy levels. At the risk of
reducing the count rate so low that statistical reliability is not
well established, an alternate embodiment is set forth wherein the
shield is of specific angular extent and omitted elsewhere. For
instance, the shield can have a width of 60.degree. and 300.degree.
be omitted; on rotation through one full revolution, the angular
location of the 60.degree. shield can be correlated to changes in
measured neutron flux accomplished at a much higher count rate;
this is desirable to increase the count rate for enhanced
statistical reliability.
As a generalization, the method and apparatus of the present gravel
pack investigative system involves a determination of gravel pack
quality, and is relatively insensitive to eccentering, and is
additionally relatively insensitive to material variations on the
exterior of the casing. Thus, the data from such a system is
primarily related to the nature of the matrix between the screen
and the casing, and therefore provides a good indication of
quality.
While the foregoing speaks very generally about the present system,
and provides something of a summary of the equipment and the method
of obtaining such a measure, there is a specific description of the
present invention set forth below in specific embodiments which
will be detailed. It is appropriate however to summarize very
generally the present apparatus as typically including a zero
spaced source and detector enabled to form a fast neutron flux
which reacts with the materials in the gravel pack and wherein back
scatter neutrons are detected by the detector. An azimuthal feature
is included derived from a rotated shield where the shield is quite
substantial with a narrow window or the reverse of the shield is
used. A second detector spaced lengthwise along the supporting
sonde is included to make base line measurements. The system relies
on back scatter from the materials making up the gravel pack and to
that extent, it is responsive to those materials in the immediate
vicinity, yielding a shallow depth of investigation which is
directed to the region where the gravel pack is located.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features, advantages
and objects of the present invention are attained and can be
understood in detail, more particular description of the invention,
briefly summarized above, may be had by reference to the
embodiments thereof which are illustrated in the appended
drawings.
It is to be noted, however, that the appended drawings illustrate
only typical embodiments of this invention and are therefore not to
be considered limiting of its scope, for the invention may admit to
other equally effective embodiments.
FIG. 1 is a sectional view through the gravel pack logging tool of
the present disclosure showing the tools suspended in a production
tubing in a well which is provided with a gravel pack and screen
located in the cased well borehole;
FIG. 2 is a sectional view through a tool detector system in
accordance with the present disclosure;
FIG. 3A is a sectional view through a detector showing a zero
spaced radiation source on the interior in conjunction with a
detector and segment of a shield thereabout;
FIG. 3B is a view similar to FIG. 3A showing the same detector with
similar shield material wherein FIG. 3B differs in that the shield
fully extends around the detector and has a small window;
FIGS. 4A and 4B show a graph of the count rate versus porosity. See
corrected FIGS.
FIGS. 5A and 5B are graphs similar to FIGS. 4A and 4B;
FIG. 6 is a graph showing the count rate as a function of a step
change in density/porosity along the axis of the well borehole;
and
FIG. 7 shows porosity apparent readings of a gamma porosity and a
neutron porosity measurement so that the two curves aid in
identifying the condition of a gravel pack in a well borehole.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Attention is now directed to FIG. 1 of the drawings where the
numeral 10 identifies a sonde which is constructed in accordance
with the teachings of the present disclosure and further wherein
the sonde supports a measuring system as described below. Before
going to the specifics of that, the location in which the sonde 10
is used should be described in conjunction with the equipment
incorporated in FIG. 1 which enables the sonde to be lowered into a
well borehole for obtaining measurements indicative of gravel pack
quality. This therefore involves a description of the supportive
equipment cooperative with the sonde, and also sets forth a
detailed description of the cased well and various aspects
regarding it.
The sonde incorporates a closed and sealed housing 12 which is
provided to operate at elevated temperatures and pressures while
protecting the equipment on the interior. The equipment on the
interior incorporates a radiation source 14 which will be described
in detail. The source may be external (like a band) around the
detector without effecting measurement. It is shown located
internally of a first detector 16, and is spaced along the length
of the tool from a second or far detector 18. The detectors 16 and
18 provide output data in the form of measured counts indicative of
the impingement of backscattered neutrons in the region of the
detectors 16 and 18. Moreover, the equipment utilizes telemetry
circuits to provide the count rates on one or more conductors which
are extended through and along a logging cable 20 which supports
the sonde 10 in the well. Typically, the sonde is lowered to the
bottom of the well and is retrieved by moving upwardly in the well.
This enables measurement of the sonde location supported on the
logging cable 20. The cable 20 passes over a sheave 22 at the
surface and is spooled onto a large storage drum 24. The cable 20
includes one or more signal conductors which provide signals to the
surface and these signals are continued from the logging cable to a
surface computer 26. Calculations by the computer 26 are output to
a recorder 28. The data is recorded as a function of depth. Depth
measurement is obtained by an electronic or mechanical depth
measuring system 30 which connects with the sheave 22 and provides
a depth measurement.
In the well, the numeral 32 identifies the casing which is held in
position with the hole in the earth's formations by a layer of
cement 34. The completed well is perforated at 36 into a producing
formation 40. There are typically many perforations. They produce
formation fluid from the formation 40 which flows through the
perforations and to the interior of the cased well. As shown, the
perforations permit this fluid flow to drain into the cased well
borehole typically flowing as a result of a positive formation
fluid drive.
There is always the risk that the formation will produce sand along
with the fluid mixture, typically, a mixture of oil and water. The
sand from the formation will flow through the perforations and
tends to plug or choke the well because the sand will typically
accumulate adjacent the zone 40 where production is achieved. As
the sand is produced, it collects in the cased well above the
packer (not shown) which defines the isolated zone. The packer
defined zone will normally accumulate the sand until the sand
completely clogs the system and prevents proper production of the
formation 40.
The well of the present disclosure is provided with an improved
production apparatus which includes a gravel pack 42. The gravel
pack is formed of gravel like material arranged in an annular space
on the exterior of a cylindrical screen 44 which holds the gravel
in place. The produced fluid can percolate through the gravel pack,
and the sand that is in the produced fluid will tend to settle
toward the bottom. The gravel pack therefore serves the desirable
purpose of providing a serpentine and multifaceted flow path for
the production fluid flow. It is not as vulnerable to silting which
might otherwise tend to plug the well. The gravel pack maintains
this protection between the perforations into the formations and
the screen 44. Generally, the screen is intended to be concentric
about the well, centered between the casing 32 and the production
tubing 46 which is arranged in the well. In similar fashion, the
sonde 10 is centralized in the tubing 46 by centralizers on the
sonde 10 the centrilizers being omitted for sake of clarity.
Ordinarily, production flows from the perforations 36 and into the
gravel pack 42. The production flow continues radially inwardly
above the bottom packer (not shown) which defines this production
zone 40. The production of fluid from the perforations 36 through
the gravel pack 42 and then through the screen 44 continues through
the production tubing 46 perforated at 48.
After a well has been operated for an interval, there may be the
risk of settling or other types of segregation in the gravel which
makes up the gravel pack. It is therefore helpful to periodically
test the well for integrity of the packing material in the well. A
loss of integrity is typically evidenced by a large void or
plugging in the gravel pack. The present apparatus is a system
which is intended to accomplish this.
As shown in FIG. 1 of the drawings, the numeral 14 identifies a
source of neutrons. These are relatively fast neutrons,
sufficiently fast that they are not detected by the detector 16
because they have energy levels which are excessive for detection
thereby. The detector 16 more aptly responds to thermal neutrons.
The numeral 50 represents a typical backscattering pathway whereby
a neutron is emitted from the source 14 and is deflected along its
pathway and returned by means of backscatter reactions toward the
detector 16. The detector 16 is at zero spacing from the neutron
source 14. By that, it is meant that both are located at a common
location. The common location is occasioned by positioning the
neutron source at the center of the detector. The detector is not
responsive to extremely fast neutrons which are emitted from the
source. Thus, in that sense, the detector is transparent to high
energy neutrons. It is not transparent however to thermal neutrons
which are returned in the backscattering approach chosen for the
present disclosure. This system is different from other types of
systems which typically utilize a forward scattering approach.
The hypothetical neutron path 50 has been exaggerated in length to
provide a representative example of this backscattering. As a
practical matter, the neutrons which are emitted from a source are
provided with energy levels great enough that the neutrons
penetrate beyond the casing 32 into the adjacent formations.
However, neutrons thermalized at this distance will not have
sufficient energy to return to the detector. There is a limited
range at which backscattering can occur. In part, that depends on
the type of materials that are in the immediate area and also
depends on the type of interaction that occurs between the
backscattered neutrons and the matrix of materials which are
irradiated by the neutron emissions. For this reason, it is
desirable to position an independent neutron measuring device which
is able to provide readings of thermal energy neutrons which are
returned from the immediate vicinity. As a generalization, the
backscatter range of neutrons emitted by the present apparatus is
only three to five inches. At ranges beyond that, it is rather
improbable that the neutrons will be backscattered and measured. As
a practical matter, this means that the responsive area is within
the casing, and it generally does not involve the regions external
to the casing. In other words, the steel which makes up the casing,
the cement which lines the well borehole and the materials which
make up the earth's formations adjacent to the well are generally
not involved in the backscatter reaction. The neutron source 14 (a
source of fast neutrons) might be Cf-252 or alternately AmBe-241.
The curves of FIG. 4 show porosity responses for the latter type
source while the curves in FIG. 5 show responses for the former
neutron source. To the extent that such a source can be adapted and
used, it is normally located at a finite point, being structurally
relatively small so that it can be located as shown in FIG. 1 of
the drawings. An alternate source is an encircling ring or band of
appropriate material. The detector is typically an He-3 detector
system.
In FIG. 1 of the drawings, both the detectors 16 and 18 are formed
of the same type detector systems, preferably being He-3 detectors,
and they typically have approximately equal size. If anything, the
detector 18 can be made larger so that it provides an increased
count rate as a result of the increase in size. This will tend to
increase the count rate to over come the reduction in count rate
which results from the greater linear spacing between the source 14
and the detector 18.
Noting that the backscatter range provides a depth of investigation
of only 3 to perhaps 5 inches, the system of the present disclosure
is able to irradiate the gravel packing materials quite readily
without obtaining data from the region beyond the casing. This
reduces the difficulties in elimination of environmental effects.
These effects are even further reduced by obtaining a recording as
a function of depth of the detector 18. Because of the greater
spacing between that detector and the source, the primary purpose
of the detector 18 is to provide a measurement which can be used to
correct the small environmental effects in data from the detector
16.
Going now to other views in the drawings, the numeral 60 identifies
a modified collimation source or system. The system shown in FIG.
2, the numeral 52 identifies the neutron source which is located
within the detector 54. Again, the detector can be a typical He-3
detector which is isolated in that region. There is a suitable gap
56 which enables the emitted neutron flux to flow out through a
steel shell 58 which defines the structure of the sonde. There are
upper and lower shielding at 62 and 64 which is preferably formed
of B.sub.4 C which serves as a collimator to direct the neutron
flux out to the gap or window at 56. This system provides a
radially outwardly directed neutron flux.
Using FIG. 2 as a representative irradiation source which provides
a flux radially outwardly into the gravel pack region, FIG. 6 shows
the vertical response of such a source as that shown in FIG. 2.
This shows that a 10% to 90% detector response is achieved in 8 cm.
for a step porosity charge of 0% to 40% (i.e. 2.65 gm/cc to 1.99
gm/cc for a matrix with density 2.65 gm/cc and fluid of 1.0 gm/cc)
along the borehole. Should the gravel pack be located further, from
the sonde, the curve would tend to be flattened and less sharply
defined. Should there be no vertical collimator, the curve likewise
would have reduced vertical resolution. In summary, FIG. 6 shows
certain aspects of the vertical response and resolutions which
might be achieved in the context of this type or extent of vertical
collimation.
Typical thermal neutron detectors, such as He-3 proportional
counters, are sensitive to detecting both thermal and epithermal
neutrons. The relative sensitivity to one or the other is
determined by gas pressure and shielding. To detect primarily
epithermal neutrons, gas pressure is increased thereby raising
epithermal neutron detection efficiency and the detector is also
surrounded by a primarily thermal neutrons, lower gas pressures are
used to reduce the portion may still be counted. FIGS. 3A and 3B
illustrate detector systems for performing azimuthally sensitive
thermal neutron detection employing a difference in technique for
removing the epithermal neutron contribution.
FIGS. 3A and 3B show detector systems which each include two
stacked cylindrical detectors 54 and a neutron source located at
the interface of these detectors. The source 52 may be positioned
at the axis of the detectors or as a band or ring around the
perimeter of the detectors at the same interdetector interface. A
single position sensitive detector can be used. In both instances,
it is preferable to utilize a motor which rotates the surrounding
shield through 360.degree. of rotation with respect to a vertical
axis coincident with the tool axis and the detectors. A motor M is
included for this purpose. It is connected to rotate the shield. As
a practical matter, the shield can be affixed to the detector and
the two can be rotated together by the motor. Radiation from the
fast neutron source is normally omnidirectional so that it has no
directional preference. Likewise an unshielded detector or one with
a uniform shield does not have a directional preference. They
respond in all directions. A directional preference defined by a
window is incorporated by placing shielding material such as
cadmium of the requisite thickness on the detector. Comparing the
two views, the construction in FIG. 3A enables the detector to
receive a higher count rate because the amount of shielding is
reduced. Since the shielding is reduced, the count rate is higher
but the angular discrimination is reduced. By rotating the shield
72 for a full revolution at a fixed elevation, it is possible to
obtain azimuthal discrimination for the detector. By contrast, the
construction shown in FIG. 3B provides a reduced count rate but
sharper azimuthal discrimination. The shield fully encircles the
detector except for the small window.
The shields can have an angular extent which can be varied. To have
a modest reduction in the direction of azimuth of interest, the
shield 72 is preferably in the range of perhaps 15.degree. to
45.degree. in arc. In one embodiment, the detector may be shielded
with a shield of up to about 75.degree. azimuthal angle. The window
in the shield 74 can be of that size. As will be understood, in
both instances azimuthal resolution is impacted by the shield and
window angular size. The advantages of the embodiment in FIG. 3A
are therefore an increased count rate but at the cost of reduced
recognition of adjacent voids in the gravel pack material while the
embodiment in FIG. 3B provides enhanced resolution but at the cost
of operating at a reduced count rate. The latter is desirable to
the extent that sharp definition is obtained so long as the count
rate is sufficiently high to have statistical reliability.
In operation, the rotated shield window mechanism shown in Figs. 3A
and 3B enables resolution of a nearby void in the gravel pack
material. This is accomplished even in face of reduced density
contrast between the packing material and the fluid which fills the
gravel pack region. Thus, there is less contrast in the advent of
gravel pack materials having a density of perhaps 1.8 as opposed to
2.65 gm/cc which had prevailed in years past. Consider as one
example, a 40% porosity fresh water sand associated with a
desirable or proper gravel pack in the cased well; neutrons emitted
from the fast neutron source are thermalized in the gravel pack
region and are backscattered to the detector. This provides a
response for one cycle of rotation of the shielding around the
detector (it being recalled that the detector functions in an
omnidirectional fashion except where shielding makes some impact;
either the shield can be rotated or both the shield and the
detector can be rotated). Simultaneously, a reading is taken from
the detector 18. The latter provides a curve, with appropriate
sizing, of the background and permits the background reading to be
deducted from the reading of the rotated detector system thereby
enabling removal of background variations during the interval of
recording the data during one revolution.
In FIGS. 3A and 3B, it is desirable to position two similar
detectors serially where the first detector in FIG. 3A has the
partial shield and the second has no shield. Likewise, FIG. 3B
shows a first detector which is a substantially shielded with a
window and the second detector has a complete shield.
Approximations of the count rates observed in the two detector
schemes shown in FIGS. 3A and 3B are a function of the surface area
of the detector and the neutron flux per unit surface area per unit
of time. The following six equations thus describe the situation
with the shield and detector arrangement shown in FIG. 3A and 3B:
using the notations C.sub.1 and C.sub.2 to describe generally the
count rate at the two adjacent detectors.
In the foregoing, C.sub.16 and C.sub.16 the count rate in the
detector 16 of FIG. 1 provided with the shield system shown in FIG.
3A or 3B respectively. The symbol A.sub.s is the surface area of
the shield and the A represents the surface area of the detector.
The symbols .PHI..sub.t and .PHI. represent the thermal and the
above thermal energy neutron flux backscattered to the surface of
the detectors. The count rate, C.sub.16 is the thermal flux in the
direction of the shield strip. The count rate C.sub.16 is the
thermal flux entering through the inshielded window. These
differences in measurements enable the thermal counts to be
separated from the epithermal. This is normally a problem because
many neutron detectors, such as He-3, are sensitive to neutrons of
both energies.
The contrast between FIGS. 4 and 5 show the difference in the
relative detected count rate from AmBe-241 and Cf-252 respectively.
Otherwise, FIGS. 4 and 5 are identical except for this change. The
contrast between FIG. 4A compared with FIG. 4B (and also comparing
FIG. 5A to 5B) shows the contrast in response for thermal and
epithermal detectors. The data indicates relatively good
sensitivity to porosity. The data shown in FIGS. 4 and 5 thus shows
that the gravel pack material provides the necessary response and
that variations in porosity can then be used to locate voids in the
gravel pack material.
FIG. 7 of the drawings shows measurements of porosity in the
ordinant with variations in gamma porosity and neutron (zero spaced
porosity). The notations across FIG. 7 show a good gravel pack, and
then a poor gravel pack. In the presence of natural gas, the curve
of FIG. 7 at 80 shows a good gravel pack while a poor gravel pack
is shown at 82. Note the difference in the readings. Finally, the
curve at 84 shows another good gravel pack indication.
The separation of the apparent porosity responses using these
measurements enables pack quality to be determined even in the
presence of natural gas or high-thermal neutron absorber
concentrations.
While the foregoing is directed to the preferred embodiment, the
scope thereof is determined by the claims which follow:
* * * * *